Cache L2ARC accesses

Cache: L2ARC accesses

The L2ARC is the 2nd Level Adaptive Replacement Cache, and is an
SSD based cache that is accessed before reading from the much slower pool
disks. The L2ARC is currently intended for random read workloads. This statistic shows
L2ARC accesses if L2ARC cache devices are present, allowing its usage and performance
to be observed.

When to check

When investigating performance issues, to check how well the current workload is caching
in the L2ARC.

Breakdowns

Breakdown

Description

hit/miss

The result of the L2ARC lookup. hit/miss states are
described in the table below.

file name

The file name that was requested from
the L2ARC. Using this breakdown allows hierarchy mode to be used,
so that filesystem directories can be navigated.

L2ARC eligibility

This is the eligibility of
L2ARC caching, as measured at the time of L2ARC access.

As described in Overhead, breakdown such as by file name would be the
most expensive to leave enabled.

Further Analysis

To investigate L2ARC misses, check that the L2ARC has grown enough in
size using the Advanced Analytic Cache: L2ARC size. The L2ARC typically takes hours, if not days,
to warm up hundreds of Gbytes when feeding from small random reads.
The rate can also be checked by examining writes from Cache: L2ARC I/O bytes. Also check the
Advanced Analytic Cache: L2ARC errors to see if there are any errors preventing the L2ARC from
warming up.

Cache: ARC accesses by L2ARC eligibility can also be checked to see if the data
is eligible for L2ARC caching in the first place. Since the L2ARC
is intended for random read workloads, it will ignore sequential or streaming read
workloads, allowing them to be returned from the pool disks instead.